Coastal circulation from along-track satellite altimetry along the northern inner shelf of the Gulf of Cadiz

Satellite-derived sea level records are generally known to be unreliable in coastal zones. However, major progresses have been made during the last decade based on improved corrections and reprocessing of along-track altimeter data. In this study, altimeter-derived CryoSat-2 products are used to study the coastal circulation along the inner northern shelf of the Gulf of Cadiz, at the southern extremity of the western Iberian upwelling system. Coastal upwelling in this region results from coastal divergence due offshore Ekman transport under westerly favourable wind, and gives theoretically origin to a cross-shore pressure gradient. Upwelling activity is usually identified based on sea temperature cooling at the coast and the development of upwelling jets produced by geostrophic balance. These eastward alongshore flows alternate with westward flows (Coastal Counter Currents, CCCs) which main driver (e.g., local wind stress, geostrophic balance or alongshore pressure variations) is not definitively identified yet.

The present research proposes to get insights into the factors that drive the coastal circulation, based on the Absolute Dynamic Topography (ADT) obtained from CryoSat-2 along-track products (in SAR mode). The studied coastal stretch, about 200 km in length, is broadly oriented E-W, allowing the use of (the meridional) satellite tracks for the determination of cross-shore sea level variations. For validation, sea level oscillations from tidal gauges are compared with sea level anomalies from nearby tracks. In general, the satellite-derived sea level data reproduce adequately the temporal trends of water level variations at the coast. However, the CryoSat-2 data obtained at less than 3/5 km from the coast was discarded to reduce potential error in the magnitude of the variations.

The coastal circulation along the coast is diagnosed based on Sea Surface Temperature (SST) satellite images and in situ Acoustic Doppler Current Profilers (ADCP) observations on the inner shelf. Remarkably, the validated cross-shore sea level data show that the water level is systematically lower near the coast during periods of active coastal upwelling. The width of the sea level gradient varies between 10 and 25 km, and closely corresponds to the cold water area identified from SST images. The corresponding geostrophic flow is estimated about 0.5 m/s, similar to the observed upwelling jets near the surface. By contrast, periods with CCCs generally correspond to a flat cross-shore slope, discarding geostrophic balance as their main driver. On-going work analyses jointly CryoSat-2 tracks which are temporally closed for the determination of sea level and slope variations along the coast associated to the development of strong alongshore flows.